1. Field of the Invention
The present invention relates to a direct current stabilization power supply apparatus which has an output short-circuit protection function.
2. Description of the Related Art
Conventionally, there are many direct current stabilization power supply apparatuses disclosed which have a protection function to protect an output transistor from a large current or a high power besides a regulator function to drive and control the output transistor to generate and output a desired output voltage. Among such direct current stabilization power supply apparatuses, there are many power supply apparatuses in which a correlation between an output voltage Vo and a drive current Ido shows a fold-back type drooping characteristic. In such a direct current stabilization power supply apparatus, when the output voltage drops, the drive current is limited according to the fold-back type drooping characteristic.
Here, a graph illustrating a general fold-back type drooping characteristic is shown in FIG. 12. As the graph shows, when the output voltage drops, the drive current is limited according to the fold-back type drooping characteristic. Conventionally, there is a problem that when the power supply apparatus is turned on with the output voltage being a negative voltage, the drive current becomes 0 and the output voltage cannot be boosted. However, JP-A-2004-348216 proposes an improvement measure to this problem.
According to this improvement proposition, a correlation between the output voltage and the drive current has a fold-back type drooping characteristic shown in FIG. 13, and a drive current flows at the time of turning on the power supply apparatus even if the output voltage is a negative voltage. Accordingly, the output voltage starts up according to the fold-back type drooping characteristic shown in FIG. 13, and the problem mentioned above is eliminated. As another document on the prior art, besides the patent document mentioned above, there is JP-A-2005-251130.
In the power supply apparatus mentioned above, when lowering the drive current to curb heat quantity at the time of shot-circuit (Vo=0 V), it is necessary to drop, for example, a voltage that is clamped at the time of short-circuit to a negative voltage. However, by dropping the clamped voltage, the fold-back type drooping characteristic in the power supply apparatus becomes substantially the same as the antecedent fold-back type drooping characteristic, and the dive current can become 0 at a small negative voltage.
Besides, if the drive current at the time of short-circuit is excessively lowered, the drive current can become 0 even at a relatively low negative voltage (e.g, −0.1 V) because of temperature changes and differences in circuit-device characteristics. Accordingly, because it becomes hard to secure a stable operation at the time of turning on the power supply apparatus, it is not desirable to excessively lower the drive current at the time of shirt-circuit.